The invention relates to multiplex radio transmission systems and more particularly to facilities in such systems for initiating protective switching from a working channel to an idle channel.
In a typical multiplex radio transmission system, a large number of voice circuits may be accommodated on one of a plurality of radio channels each occupying a frequency spread of about 20 MHz. Most of the radio channels of the multiplex system are employed as working channels; however, a certain number, usually in a ratio of 1:10 to the working channels, are reserved as idle channels. Signals on a working channel are selectively switched to an idle channel via a protective switching apparatus when the level of a prescribed spurious component on the channel, relative to that of the desired signal level thereon, increases above a predetermined value.
Existing protective switching facilities of this type generally operate in dependence on the relative level of baseband thermal noise on the associated working channel. When the desired information signal on the channel is sufficiently attenuated (e.g., in the presence of atmospheric disturbances such as multipath fading and rain attenuation), the thermal noise level relative to the signal rises substantially from its normal value (e.g., at least -50 dB) measured after down-conversion to baseband. As soon as the signal-to-noise ratio under such conditions drops to about 15 dB, the voice circuits on the affected radio channel are switched to an idle channel by means of the protective switching facilities.
In order to reliably operate such switching facilities in the past, it has been necessary to fairly accurately determine the relative noise level at baseband. Because it is extremely difficult to measure baseband thermal noise directly in the presence of signal, and further because such thermal noise characteristically increases in square law fashion with frequency, it has been common to measure the noise level at a frequency far enough above baseband so that there is no appreciable signal amplitude to interfere with the measurement. The so-measured noise level is then extrapolated downward to baseband to approximate the relative noise level for purposes of establishing the level for channel switching.
Unfortunately, when the desired signal component on the working channel is also accompanied by interference (e.g., crosstalk between channels), the determination of the threshold level for channel switching initiation is difficult because such interference does not obey the square law of baseband noise.